An autopsy late Tuesday revealed that the child died of a lacerated liver brought on by blunt force trauma to the abdomen.
Police say the injuries caused bleeding elsewhere and that initially suggested evidence of sexual abuse.
But the autopsy revealed the girl had not been sexually assaulted.
Police sources tell Action News the child arrived at Children's Hospital of Philadelphia around 2:10 a.m. Tuesday.
Doctors saw the child had bruising, both old and new, all over her body and was bleeding from the ears and other areas.
She was pronounced dead at 2:45 am.
Police and DHS were notified.
Sources say the 4-year-old girl lived at a home on the 2200 block of South 56th Street with her mother, her mother's boyfriend and two siblings.
Special Victims Unit took the girl's mother and her live-in boyfriend in for questioning. They were released Monday pending the outcome of the autopsy and further investigation.
Neighbors said the couple moved onto the block about 3 months ago.
They saw a good deal of police activity - including the crime scene unit at the home on Monday.
"Police have been here since last night. They have been running in and out of the house with bags taking pictures," neighbor Manny Yates said.
Sources say DHS has taken an 11-month-old from the same address to CHOP to get checked out.
The age of the other sibling has not been released.
DHS is continuing to investigate.
No charges and no arrests have been announced.